This year appears set to be the coolest globally this century.

cold 2008
The early part of 2008 saw continued low temperatures in some regions

Data from the UK Met Office shows that temperatures in the first half of the year have been more than 0.1 Celsius cooler than any year since 2000.

The principal reason is La Nina, part of the natural cycle that also includes El Nino, which cools the globe.

Even so, 2008 is set to be about the 10th warmest year since 1850, and Met Office scientists say temperatures will rise again as La Nina conditions ease.

"The big thing that's been happening this year is La Nina, which has lowered global temperatures somewhat," said John Kennedy, climate monitoring and research scientist at the Met Office's Hadley Centre.

"La Nina has faded in the last couple of months and now we have neutral conditions in the Pacific," he told BBC News.

Scientists at the World Meteorological Organization have also suggested that 2008 will turn out to be cooler than the last few years.

Breaking the ice

La Nina cools waters in the eastern Pacific Ocean, but its effects are felt around the globe.

It is one of a number of natural climatic cycles that can re-inforce or counteract the warming trend stemming from increased levels of greenhouse gases in the atmosphere.

Earlier this year, one group of researchers suggested that another natural cycle, the Atlantic Multidecadal Oscillation, was likely to hold temperatures steady for about the next decade, before reversing direction and allowing a renewed warming.

"The principal thing is to look at the long-term trend," said Dr Kennedy.

"2008 will still be significantly above the long-term average. There's been a strong upward trend in the last few decades, and that's the thing to focus on."

One of the starkest effects of rising temperatures has been the rapid loss of summer Arctic sea ice, which has accelerated since the year 2000.

Earlier in the year, there were indications that 2008 could see even more ice lost than in the record-breaking melt of 2007.

Currently, the ice appears to be holding together better than a year ago, although scientists are wary as much of it is relatively fragile ice that formed in a single winter.

Canadian authorities have just declared that the Northwest Passage is "navigable", though acknowledging that some parts of it still contain floating ice.